The overall objective of the proposed research is to attain a more complete understanding of mitosis in order to find ways to specifically arrest cells in mitosis. Human, hamster and human-hamster hybrid cells will be used as the main source of experimental material. Cell culture, cytological, ultrastructural, and biochemical methods will be used in the experiments. The specific objectives of the research include: 1. Further characterization of temperature-sensitive mutant hamster cell lines in which mitosis is defective at the non-permissive temperature. 2. Biochemical and ultrastructural analyses of the mutants to determine the specific gene products involved in the mutations. 3. Elucidation of the mechanism for chromosome movement. 4. Isolation of additional mutants blocked in mitosis for defining other gene products essential for the successful completion of mitosis. 5. Performance of genetic analysis with the mutants, including the utilization of human-hamster hybrid cells formed between mutant and human cells.